Multicast where a packet is transmitted to a plurality of destination terminals is conventionally performed (“Deploying IP Multicast in the Enterprise” by Thomas A. Maufer, translated by Hiroyuki Kusumoto). In a communication system, multicast is performed by use of protocols whose standardization is being promoted by the Internet Engineering Task Force (IETF), the protocols being such as: Source-Specific Multicast (SSM) (Internet Draft, “dradt-ietf-ssm-overviw-xx.txt”, “Japanese Journal B of the Institute of Electronics, Information and Communication Engineers”, Vol. J85-B, No. 8, pp. 1207-1214); Internet Management Protocol Version 3 (IGMPv3) (RFC3376, “Internet Management Protocol Version 3”); Hop by Hop Multicast Routing Protocol (HBH) (L. HMK Costa, S. Fidia and O CMB Duarte, “HOP by HOP Multicast Routing Protocol”, ACM SIGCOM 2001, August 2001); Protocol Independent Multicast-Sparse Mode (PIM-SM) (RFC 2362, “Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification”).
Furthermore, in order to continue communications even when a source terminal transmitting a multicast packet moves and then its address is changed, Mobile IP Bi-Directional Tunneling (MIP-BT) is being proposed. In MIP-BT, communications are intended to be continued by forwarding a multicast packet transmitted from a source terminal which has moved to a foreign network via a home agent.